OF THE CRANIUM. 55 



These bones arc of a flattened form. They are composed of 

 two lamina or plates called tables, with a cellular structure 

 between them, called meditullium, or diploe. The external 

 table is more firm and thick than the internal. The latter is 

 comparatively" very brittle, whence it is called the vitreous 

 table. [Between the two tables which compose the flat bones 

 of the cranium and running through the diploe are several 

 sinuses, which are occupied by veins in the recent subject. 

 They were discovered by M. Fleury about forty years ago, 

 while he was Prosector at the School of Medicine in Paris, and 

 engaged in some inquiries relative to the structure of the 

 cranium at the instigation of M. Chaussier. The account 

 which M. Chaussier gives of these veins is as follows : they are 

 situated in the middle of the diploe between the two tables of 

 the skull, and like all other veins are intended to return the 

 blood to the heart. They are furnished with small valves, 

 have extremely thin and delicate parietes, and commence by 

 capillary ramifications coming from the- difl^erent points of the 

 vascular membrane which lines the cells of the diploe. Their 

 roots are at first extremely fine and numerous, form by their 

 frequent anastomoses a kind of network, and produce by their 

 successive junction, ramuscles, branches, and large trunks, 

 which, becomino- still more voluminous, are directed towards the 

 base of the cranium. Some varieties exist in regard to the 

 number, size, and disposition of these trunks, but generally one 

 or two of them are found in each side of the frontal bone, two 

 in the parietal bone, and one in each side of the occipital 

 bone. Anastomoses exist between these several trunks, by 

 which the veins in the parietal bone are joined to those in the 

 frontal and in the occipital. Branches from the right side of 

 the head also anastomose with some from the left side. Besides 

 the branches already mentioned, one or two smaller than the 

 others are directed towards the top of the head and terminate in 

 the longitudinal sinus. 



The descending veins of the diploe communicate in their 

 passages with the contiguous superficial veins, and empty into 

 them the blood which they receive from the several points of 



